While the fetal cardiac output is several fold higher than the adult, the factors that regulate fetal cardiac output remain largely unknown. These include peripheral factors affecting venous return as well as central factors affecting the heart. This study is designed to determine the importance of venous return in regulating cardiac output in the fetus. The study will begin by describing in the normal fetus venous return, mean systemic pressure, the venous return curve, resistance to venous return and vascular compliance. Next the effects of clamping the umbilical cord, increased fetal arterial PO2, hypoxia and bradykinin will be studied as they affect to these same parameters. Both chronic and acute experiments are planned in fetal sheep to measure venous return and cardiac output and Pms. An extracorporeal perfusion system, bypassing the fetal heart, will be used to measure mean systemic pressure vascular compliance and the venous return curve. Compliance will be measured by injecting a known volume of blood and observing the change in mean systemic pressure. Venous return curves will be obtained and compared to cardiac function curves. Fetal cardiac output is important to the delivery of oxygen and nutrients for normal growth and development, and maintenance of an adequate cardiac output during birth is important in sustaining vital areas, such as the brain and heart.